Electrification of large drops by collision with small drops in an electric field.—The following experiments were made to test Elster and Geitel's theory of the electrification of rain drops. Large drops (A drops) falling between the horizontal parallel plates of a charged condenser, were sprayed from below with a fine stream of small drops (B drops) and the charge on the A drops was found to be of the same sign as that of the upper plate and to be proportional to the field strength. From the charge per A drop and the probable number of collisions with B drops, the charge per collision was computed and found to be zero for B drops of radius less than 0.18 mm and then to increase rapidly with the radius to 0.4 mm. Photographs taken with a motion picture camera show that the B drop first coalesces with the A drop and then, if its momentum is sufficient, a protuberence is produced on the upper surface of the A drop, which breaks up into a number of small droplets which carry off with them not only part of the A drop but also some charge. Very small B drops probably merely coalesce with A drops. In the case of falling rain, cloud droplets are too small to have a charging effect, and the effect of larger drops overtaking smaller ones is to weaken the existing field. Hence it is doubtful whether this phenomenon alone can explain the observed charges on raindrops and it certainly does not explain such high potential differences as occur in lightning discharges.